Brushless direct current electric motor with reduced cogging torque and production method thereof

ABSTRACT

The invention relates to a brushless direct current motor, having a rotor made up of at least one permanent magnet and a stator having at least three partitions (160) radially extending from a circular based cylindrical main body (170), the partitions (160) together defining at least two volumes for receiving at least three coils generating a magnetic field, wherein each volume is closed by a wall (170) connecting the partitions (160), and in that the wall comprises, on the face thereof oriented toward the rotor, at least one magnetic restriction zone. A sleeve (4) surrounds the stator and the rotor and has at least one deformation zone formed by cutouts (11) adapted to maintain the external geometrical configuration of the sleeve (4) when mounting the constituent elements of the motor. The invention also relates to a method for manufacturing such a motor.

BACKGROUND OF THE INVENTION

The present invention relates to a brushless direct current electricmotor with reduced cogging torque. Such cogging torque is also referredto as notching torque. For the sake of readability, the term “cogging”will be preferably used hereafter.

The invention also relates to a method for producing such a motor.

A brushless direct current motor is commonly referred to as a brushlessmotor. Such a motor comprises a rotor, made up of at least one permanentmagnet, and a stator comprising a plurality of coils generating amagnetic field. The coils are powered sequentially. In this way, amagnetic field is generated that rotates at the same frequency as thepower supply voltages of the phases. The permanent magnets) of the rotorattempt to be oriented at each instant in the direction of the magneticfield, which leads to the rotational movement of the rotor. The supplyvoltages of the phases are continuously adapted so that the fieldremains in advance of the position of the rotor, in order to obtain amotor torque. A brushless motor converts electric energy into magneticenergy and then into a mechanical rotation movement.

Some brushless motors are configured with the rotor positioned aroundthe stator. This configuration allows high motor torque to be obtainedwith a relatively low rotation frequency.

Other types of brushless motor, which are more common, have a mirrorconfiguration, namely a rotor inserted into the stator. In this case,the rotation speeds are higher for a lower torque, for an equivalentsize of motor. Rotor-in-stator type brushless motors are used in somefields where torque must be provided in a limited space, with safe andcontinuous operation. They are found, for example, in the aeronauticalfield for electromechanical actuators.

Hereafter, for the sake of readability, the invention will be describedfor a brushless motor with the rotor inserted into the stator, with itbeing understood that the invention is also applicable in the mirrorconfiguration, namely with a rotor surrounding the stator.

In the illustrated configuration, the stator forms a circular basedhollow cylinder with a central volume that receives the rotor, which isalso in the form of a solid, circular based cylinder. The rotorcomprises a shaft for driving a defined component. Radial partitionsextend between the central volume and the outer wall of the stator. Thepartitions together define unitary volumes or notches, which are opentoward the rotor. De facto, these notches surround the rotor when saidrotor is introduced into the central volume of the stator. Thus, as aradial section, the stator has a star shape or, more precisely, astar-shaped ring.

These notches enable the winding to be carried out, namely the windingof the constituent copper wires of the electromagnetic coil of thestator. Therefore, in a cross section of the stator, an alternation ofsolid zones, in this case the metal partitions, and of open zonesexists.

The operation of the brushless motors, irrespective of theirconfiguration, generates a particular torque called cogging torque, dueto the permanent and continuous offset between the magnetic field andthe orientation of the magnets, when the rotor rotates relative to thestator.

By way of a reminder, according to a definition that has been acceptedby the International Electrotechnical Commission (IEC), the torquereferred to using the term cogging is the cyclic torque of a permanentmagnet electric motor resulting from the tendency of the rotor and ofthe stator to align in a position corresponding to the minimum magneticreluctance.

In other words, it involves the phenomenon of stepping type rotationresistance that is observed when the rotor rotates relative to thestator. Indeed, when the magnets are facing a metal part with aprotrusion, a magnetic connection is established and afterwards there isresistance to the rotation of the rotor. In other words, the magneticfield passes through the metal zone where the air volume is the lowest.

In certain fields, such a phenomenon can at least result in a hindrancefor the user of an apparatus or device implemented by such a motor. Thisis the case, by way of a non-limiting example, in the aeronautical fieldwhere permanent magnet electric motors ensure that the flight controlsare maneuvered. Any hindrance or resistance in the handling of a flightcontrol that is felt by the pilot can affect the control of theaircraft. In other words, fluid and continuous maneuverability needs tobe maintained, without resistance, for the flight controls, with itbeing understood that any resistance can be interpreted by the pilot asa problem on a flight component.

In order to overcome this phenomenon of cogging, while maintaining thefeatures and performance levels of a brushless motor as far as possible,closed rotor notches and/or notches with more or less complex shapes,and/or walls with complex shapes and/or stator partitions positionedoffset relative to the poles of the magnets are known from documentsUS-A-2018020923, US-A-2018019648, U.S. Pat. No. 6,784,582,US-A-2005067913, US-A-2007273234, WO-A-2018008328, for example. Thesesolutions aim to reduce cogging as far as possible.

More generally, one of the known and frequently used solutions foroptimizing cogging, i.e. to reduce said cogging while affecting theperformance levels of the motor as little as possible, involveslaterally closing the notches or at least reducing the opening to aminimum: in this way, a volume defined by solid, or practically solid,walls is created and there is a continuity, or a quasi-continuity, ofmaterial, resulting in a magnetic field without any discontinuity, andtherefore without any significant alternation of solid and empty ones.It is understood that, when the rotor surrounds the stator, there isalso a solid continuous wall between the stator and the rotor. In thiscase, the closed wall is oriented toward the outside of the stator.

It is understood that such a solution is not easy to implement, inparticular because the winding is carried out manually. In other words,the lack of an opening for the rapid passage of the winding wiresaffects productivity. Indeed, the copper wires are wound by successivepassages through the end openings of the notches, by manual “knitting”.In the case whereby a minimum opening is preserved in order to be ableto carry out the winding, an opening needs to be found having a widththat is such that it enables winding, while optimizing the reduction ofthe cogging torque. Moreover, the method for closing notches alsoaffects the performance levels of the motor, especially since closingthe notches is not easy to implement.

Document US-A-2016/315508 discloses a brushless motor comprising astator surrounding a rotor. On the stator, partitions are provided andare connected by a wall. Said wall comprises, on the face thereof facingthe rotor, a magnetic restriction zone produced by a counterbore and bya material having magnetic properties different from those of the restof the stator.

It is known that the stator is manufactured by stacking rings withradial reliefs coplanar to the main plane of the ring. These reliefsdefine the partitions delimiting the notches. The notches are closed byan added part, from the internal annular space of the stacked rings, orby being surrounded with a cylinder interposed between the rings and theouter sleeve of the stator, there is an additional thickness that canaffect the magnetic field and/or the air gap in the case whereby therotor is housed in the stator.

It is these disadvantages that the invention more particularly intendsto overcome by proposing a brushless type motor with a limited coggingtorque, which has an easy and simple construction, while having littleor no effect on the performance levels of such a motor.

SUMMARY OF THE INVENTION

To this end, the invention relates to a brushless direct current motor,comprising a rotor made up of at least one permanent magnet, and astator comprising at least three partitions radially extending from acircular based cylindrical main body, said partitions together definingat least two volumes for receiving at least three coils generating amagnetic field, each volume being closed by a wall, formed by a portionof the body, connecting said partitions, and said wall comprising, onthe face thereof that is oriented toward the rotor, at least onemagnetic restriction zone, characterized in that the magneticrestriction zone is formed by at least one thinner zone of the wall, andin that a sleeve surrounds the stator and the rotor, and in that saidsleeve comprises at least one deformation zone formed by cutouts andadapted to maintain the external geometrical configuration of the sleevewhen mounting the constituent elements of the motor.

Thus, the presence of a magnetic restriction zone allows the magneticfield to be guided so that the field lines preferably pass through therotor magnet and not through the walls of the notches of levels thestator. This forces the magnetic field to continuously pass through allthe magnets, which reduces the effect of cogging as much as possible, byavoiding zones with a high volume of air.

According to advantageous but non-compulsory aspects of the invention,such a motor can comprise one or more of the following features:

the thinner zone is defined by a counterbore or a groove provided in thewall;

the magnetic restriction zone is formed by at least one wall portionmade of at least one slightly magnetic or non-magnetic material.

The invention also relates to a method for manufacturing a motoraccording to the invention, characterized in that it comprises at leastthe following steps:

-   -   a) stacking metal rings previously machined and configured        according to the cross section of the stator with partitions        delimiting open volumes for receiving the coils, the stack being        produced over a height corresponding to the desired length of        the stator;    -   b) winding copper wires around the partitions on the stack        obtained in step a);    -   c) mounting, using the lacing technique, the stack obtained in        step b) in a sleeve, with angular indexing by lugs and notches        enabling the stack to be positioned relative to the sleeve;    -   d) inserting the rotor into the volume for receiving the stator;    -   e) forming the part of the copper wires apparent on the ends or        the coil;    -   f) final overmolding of the motor elements using resin.

Such a method allows precise mounting, without generating additionalthickness by virtue of the lacing technique, which is per se known.Lacing enables a slightly tight adjustment between the stacks and themotor body, thus promoting magnetic and thermal conductivity between theelements.

According to advantageous but non-compulsory aspects of the invention,such a method can comprise one or more of the following steps:

-   -   after step e) and before step f), an additional step g) of        grinding the external and internal diameters of the stator is        carried out;    -   the sleeve is obtained by stacking rings;    -   the stacking of rings constituting the sleeve is carried out        independently of steps a) to f).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be better understood and further advantages thereofwill become more clearly apparent upon reading the following descriptionof a plurality of embodiments of the invention, which is provided by wayof a non-limiting example and with reference to the accompanyingdrawings, in which:

FIG. 1 is a perspective view of a brushless motor according to oneembodiment of the invention;

FIG. 2 is a cross section view along II-II of the motor of FIG. 1 ,without the stator and to a different scale;

FIG. 3 is a perspective view, to a different scale, of the constituentsleeve of the stator of the motor of FIG. 1 ;

FIG. 4 is a perspective view, to a larger scale, of a ring forming, bystacking, the constituent active part of the stator of the motor of FIG.1 ;

FIG. 5 is a front view of a ring similar to that of FIG. 4 , to a largerscale, according to another embodiment; and

FIG. 6 is a simplified cross section view, to a different scale, of abrushless motor without the rotor, with the ring of FIG. 5 inserted intothe sleeve of FIG. 3 .

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a brushless direct current motor 1 commonlydesignated by the term “brushless motor”.

Such a motor is made of metal material and/or composites, at least forthe active part. The rest of the motor, typically the constituentelements of the outer covering, can be made of composite materials orpolymers.

In this case, the motor 1 is a motor according to an embodiment of theinvention in which a rotor 2 is inserted into a stator 3. An outersleeve 4 surrounds the active part of the stator 3 and defines the outerwall of the motor 1. Such a motor 1 is used in various technical fields,for example, in the aeronautical, space, medical, automotive, marine,agri-foodstuff or other industry for operating a movable componenteither rotationally or translationally. In this case, a rotational driveshaft 5 is directly fixed on one end of the rotor 2 and extends saidrotor.

The rotor 2 is provided, on the outer face thereof, with at least twopermanent magnets. The magnets are, for example, glued on the outerface, with a glue bridge being provided between two neighboring magnets.The magnets extend over the entire length of the rotor 2. The magnets,the number of which is adapted to the size of the rotor and the motor,are disposed so that the north and south poles of two neighboringmagnets are alternately oriented toward the outside and toward theinside of the rotor 2. In one embodiment, not shown, the rotor 2 ishollow. As a variant, as shown in FIG. 2 , the rotor 2 is solid. Themagnets, under the effect of the magnetic field generated by the stator3, ensure the rotation of the rotor 2 and therefore of the shaft 5.

FIG. 3 illustrates the outer sleeve 4 ensuring the protection of theactive parts of the stator 3, and therefore of the rotor when said rotoris introduced into the stator opening. The sleeve 4 has a flat andunified outer face 7. As a variant, it has a different appearance and/orit comprises an informative marking. The inner face 6 of the sleeve 4comprises longitudinal grooves 8 or counterbores, with a rounded bottom,extending over the entire length of the face 6 of the sleeve 4. Thesleeve 4 is also provided, on the outside of one of the ends thereof,with at least one lug 9 and at least one notch 10 defining an indexingand guiding component when positioning the sleeve 4 on the active partof the stator 3.

Moreover, a deformation zone is provided on the sleeve 4. In this case,it is formed by a plurality of cutouts 11 provided in the thickness ofthe wall 12 of the sleeve 4. This deformation zone is adapted to absorbthe deformation of the active part of the stator 3 in the shape of astar, which is inserted into the sleeve, and thus maintain the outercircularity of the sleeve 4.

The active part of the stator 3 is the part of the stator 3 providedwith the winding and generating the magnetic field. The active part ofthe stator is formed by the active parts of the constituent rings 15 bystacking the stator. Such an active part is, by analogy with FIG. 4 ,cylindrical, hollow and has a circular base. The dimensions and theshape of the inner opening 14 of each ring 15 are adapted to accommodatea portion of the rotor 2, while ensuring its free rotation, with aminimum amount of friction.

Several partitions 16 radially extend toward the outside of each ring 15from the annular body 17 of the ring 15. These partitions 16 areconfigured as a rectangular and flat tab. The length of the partitions16 is equal to the length L of the ring 15 and they are parallel to thislength. The partitions 16 are evenly disposed on the annular body 17. Inthis case, the space between two neighboring partitions 16 is constant.As will become more clearly apparent in FIG. 6 , two neighboringpartitions are thus connected by a wall formed by a portion of theannular body. Thus, a portion of the volume for receiving the coil isdefined between the partitions and the portion of the annular body.

A longitudinal groove 18 is provided, on the inner face 21 of theannular body 17, between two neighboring partitions 16. The opening ofthis groove 18 is oriented toward the inside of the body 17. A groovingor counterbore 19 is positioned on the outer face 20 of the annular body17, facing the groove 18. The groove 18 and the counterbore 19 produce athinner zone.

As is apparent from FIG. 4 , the grooves 18 and the counterbore 19 areevenly distributed on the body 17, along the entire length L of the ring15. The grooves 18 and the counterbore 19 are produced by removingmaterial on the body 17. This reduction in the nominal thickness of thebody 17 in a given zone, and therefore once the stacked rings 15 havedecreased the active part of the stator 3, results in a magneticrestriction.

FIG. 5 illustrates another embodiment of a ring 150 similar to the ring15. In this case, only the grooves 180 are present, the grooving or thecounterbore on the outer face of the body 170 are absent. In all cases,the grooves 180 ensure, like the grooves 18, a reduction in thethickness of the active portion of the stator when the rings 150 arestacked, and therefore define a magnetic restriction zone.

In another embodiment, not shown, the stack is made from rings that areequivalent, for example, to the combined features of a ring 15 and 150.

As is apparent from FIG. 6 , when the stator 3 is mounted, and thereforewhen the active part is wound and is inserted into the sleeve 4, thegrooves 180 are substantially located in a central position between twoneighboring partitions 160, with the winding, not shown, being in place.As shown in FIG. 6 , the presence of grooves 180, due to the smalleramount of magnetic material at this point, directs the magnetic field sothat it does not pass directly from one partition 160 to another, butfollows a circuit through one of the magnets of the rotor, with saidrotor being introduced into the volume V defined for the opening of thestacked rings 150. The rotor is then set into rotation, with its magnetsattempting to align with the magnetic field. Due to the presence of thegrooves 180, and therefore of the magnetic restriction zone over theentire circumference of the body 170, an even and constant circuit isbrought about that enables cogging to be avoided. Moreover, the presenceof cutouts 11 in the sleeve 4 provides a perfectly circularconfiguration for the inside of the sleeve, thus avoiding anydeformation of the stacked rings, in particular in the vicinity of thepartitions. This also helps to ensure even rotation of the rotor.

It is understood that the operation is identical with the rings 15stacked according to the embodiment shown in FIG. 4 .

As a variant, in another embodiment, the magnetic restriction zone isobtained by a change of material. Typically, one or more materials areused with magnetic conductivity that is lower than that of the rest ofthe body 17; 170 and partitions 16; 160. It is also possible to usenon-magnetic materials.

Moreover, the grooves 18; 180 allow, due to the method for producing thestator 3, a precise adjustment to be achieved, with optimum contactbetween the parts to ensure thermal and magnetic conductivity, withoutdeformation of the constituent parts.

The mounting of such a motor will now be described with reference to thevarious figures. To this end, a stack of rings 15; 150 is initially madethat constitutes the active part of the stator 3. These rings 15; 150were previously machined and configured, according to the cross sectionof the stator to be obtained. The number of rings to be stacked dependson the desired length of the stator 3. Once the stack is made, thewinding is carried out by winding the copper wire around the partitions16; 160 from the open zone. Such winding is easy to produce, since thereis no obstacle hindering handling.

Once the winding is carried out, a stack of circular rings, notillustrated, constituting the sleeve 4 is produced. It is understoodthat the stack for the sleeve is independent of the rest of the methodand that it advantageously can be carried out at another location and/orat another time than simultaneously with the winding. In other words, itis possible to produce the sleeve 4 by stacking a number of rings thatis defined according to the length of the sleeve to be obtained,independently of the mounting of the active part and the winding. It isunderstood that, when the winding is carried out, it is important thatthe volumes receiving the coils are closed quickly by installing thestack of rings constituting the sleeve.

The previously obtained active part is then inserted into the sleeve 4using the lacing technique. By way of a reminder lacing refers to theassembly of two parts with a tight fit. Assembly is carried out byheating the outer part or hoop, which allows the cold part to be lacedto be introduced into the hoop or into a housing provided in the hoop.The adjustment between the parts is made by cooling the parts to thesame temperature. Such a technique allows a homogeneous and similaradjustment at any point in the zone for assembling the parts together.

Positioning the parts during lacing is facilitated by the presence ofthe lugs 9 and notches 10, which allow indexed positioning of the parts.

The grooves 18; 180 also allow, during temperature variations, the partsof each ring 15; 150 to deform, and thus adjust, independently of eachother, in a resilient manner.

The deformation zones 11 of the sleeve 4 mentioned above also help todefine resilient behavior of the sleeve 4. This results in reducedstresses and deformations on the entire stator 3.

The mounting of the motor is finalized by shaping the copper wireportions of the winding that appear on the ends. This step is referredto as coil production.

A final step involves overmolding all the elements of the stator and ofthe rotor with a protective resin.

The invention claimed is:
 1. A brushless direct current motor,comprising: a rotor made up of at least one permanent magnet disposed onan outer face of the rotor, wherein the permanent magnet extends over anentire length of the rotor; a stator inside which the rotor is inserted;and a sleeve surrounding an active part of the stator and defining awall, wherein the active part of the stator is the part of the statorprovided with a winding and generating a magnetic field; wherein: saidstator has a plurality of partitions extending radially outwardly from acylindrical main body, said partitions together defining at least twovolumes for receiving at least three coils generating a magnetic field,each volume being closed by a second wall formed by a portion of saidmain body connecting said partitions, wherein each of the at least threepartitions includes a flat end face for engaging an inner surface of thesleeve; said sleeve surrounds said stator and said rotor; and saidsleeve comprises: a plurality of deformation zones spaced around thecircumference of the sleeve, the plurality of deformation zones adaptedto maintain an outer circularity of said sleeve when mounting saidstator in said sleeve when said end faces of the plurality of partitionsof said stator are positioned adjacent associated ones of said pluralityof deformation zones, wherein the plurality of deformation zones areformed by a plurality of cutouts provided in the thickness of the wall;and a plurality of counterbores in the inner surface of the sleeve, eachof said plurality of counterbores having a rounded bottom and extendingover the length of an inner face of the sleeve, wherein only one of saidplurality of counterbores is disposed between adjacent ones of saidplurality of deformation zones.
 2. The motor according to claim 1,wherein said second wall formed by said main body of said statorcomprises a thinner zone defined by a grooving provided in said secondwall.
 3. The motor according to claim 1, wherein said second wall formedby said main body of said stator comprises a magnetic restriction zoneformed by at least one wall portion made of at least one slightlymagnetic or non-magnetic material.
 4. The motor according to claim 1,wherein said second wall formed by said main body of said statorcomprises a magnetic restriction zone formed by at least one zonethinner than said second wall.
 5. The motor according to claim 4,wherein said magnetic restriction zone is formed on an inner face ofsaid second wall oriented toward said rotor.
 6. The motor according toclaim 4, wherein said thinner zone is produced by a grooving.
 7. Themotor according to claim 4, wherein said thinner zone is produced by agroove on an inner face of said main body of said stator and a groovingpositioned on an outer face of said main body of said stator.
 8. Themotor according to claim 1, wherein said rotor is positioned within saidstator.